Dc protection and control panel with generator field excitation control

ABSTRACT

A system for controlling the operation of and protecting a DC generator alone and in a parallel system of DC generators. The system performs the general control functions of voltage regulation, generator paralleling, load equalization, and the general protection functions of fault detection, overvoltage protection, and including the taking of appropriate action such as disconnecting or preventing the disconnection of the controlled generator from a parallel system.

United States Patent Inventor David L. Eiaenatadt Warrenavilie Heights, Ohio Appl. No. 24,091 Filed Mar. 31, 1970 Patented Dec. 28, 1971 Assignee Lear Siegier, Inc.

Santa Monica, Calif.

. Continuation-impart of application Ser. No.

687,667, Dec. 4, 1967. This application Mar. 31, 1970, Ser. No. 24,091

DC PROTECTION AND CONTROL PANEL WITH GENERATOR FIELD EXCITATION CONTROL 9 Claims, 1 1 Drawing Figs.

US. 307/84, 322/28, 322/73 Int. Cl 1102p 9/30 Field of Search 322/28, 36, 59, 60, 73; 307/84 [56] Relerences Cited UNITED STATES PATENTS 3,209,234 9/1965 Bridgeman et al 322/28 3,214,599 10/1965 Wellford 322/28 X 3,231,757 l/l966 Rainer et a]. 322/28 X Pn'mary Examiner-Benjamin Dobeck Assistant Examiner-H. Huberfeld Attorney-Bosworth, Sessions, Herrstrom & Cain ABSTRACT: A system for controlling the operation of and protecting a DC generator alone and in a parallel system of DC generators. The system performs the general control functions of voltage regulation, generator paralleling, load equalization, and the general protection functions of fault detection, overvoltage protection, and including the taking of appropriate action such as disconnecting or preventing the disconnection of the controlled generator from a parallel system.

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Array/v5 so DC PROTECTION AND CONTROL PANEL WITII GENERATOR FIELD EXCITATION CONTROL CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of application Ser. No. 687,667, filed Dec. 4, 1967.

BACKGROUND OF THE INVENTION This invention relates to control and protection of direct current generators including DC generators of the type which may be operated in parallel in an aircraft electrical system. Operation of such generators and generator systems includes control of local generator output voltage, equalization of the load among the several generators on the line and protection against current flow from line to generator. Such a control and protection system accomplishes these and other functions by controlling generator field excitation, connection of the local generators to the main load bus, and connection of the local generators to the equalizing bus.

In the past, control circuits for direct current generators have employed control and sensing elements which are rather large and heavy for airborne use. Voltage regulation of a DC generator, for example, is accomplished conventionally by a carbon pile regulator connected to regulate the generator field excitation. Such a device normally consists of a pile of carbon discs, the total resistance of which varies inversely with the degree of compression applied. A proportioning-type electromagnetic actuator responds to the output voltage of the generator and controls the amount of compression, thereby varying proportionately the resistance of the carbon pile. As a result, more or less current flows to the exciter field of the generator and increases or decreases the generator output voltage. This type of regulator is heavy and relatively slow acting. Furthermore, it is inefficient because it is required to dissipate considerable wattage in its resistance.

Similarly, conventional controls for providing load equalization, overvoltage protection, differential protection and main contactor control are normally accomplished by large expensive, slow acting, specially constructed relays.

For example, connection of the local generator of a parallel generator system to the main load bus is generally accomplished by a main line contactor. Before a local generator can be put on the line, however, there should be no undesirable voltage difl'erence between the main load bus and the local generator. Once the local generator is connected to the main load bus, it is necessary to sense and indicate the fiow of any reverse current to the local generator indicating its having become a load on the parallel system instead of a contributor to it. In the past, separate circuits or a special complex relay has been required to sense voltage difierence between the main bus and the local generator and to sense reverse current flowing to the local generator after connection to the main bus. Such a relay, however, is expensive, heavy and extremely delicate.

Also, the means for disconnecting the local generator from the parallel system in case of overvoltage or a ground fault of some type is conventionally a specially built relay adapted to sense the overvoltage or the difference in current in different parts of the system caused by the ground fault. Such a relay normally is also heavy, expensive, and delicate. In all the specially built relays mentioned above and used for disconnecting the local generator from all or part of the system, a built-in weighting factor is required to determine that the sensed conditions actually exist and continue for a predetermined length of time and that a mere transient signal has not been sensed. These relays are required, in effect, to integrate a sensed condition over a small period of time to determine that it actually exists. This requirement adds to the size and expense of such special relays.

To a limited extent, the disadvantages of size and weight associated with the electromechanical control devices referred to above have been reduced through the use of elemental solid state devices such as power transistors, silicon-controlled rectifiers, and Zener diodes. Such savings are achieved, however, at a sacrifice in mechanical simplicity exchanged for elaborate circuits designed to utilize the solid-state devices as control and sensing elements. Furthermore, when such solidstate devices are used and operated in their active region as proportional controllers, they are required to dissipate wattage as heat. Excessive heat can cause thermal runaway and require large heat-absorbing surfaces or cooling devices for acceptable operation. Solid-state devices thus olTer only a partial solution to some of the problems of the electromechanical devices.

Solid-state devices such as those mentioned above sometimes can be operated as switches rather than proportioning devices which reduces the amount of power they are required to dissipate and thereby increases their efficiency. One type of regulator that uses this principle depends upon the generators ability to follow load conditions and switches the field on and off as required to maintain desired terminal voltage. Under such conditions, for example, a DC generator is controlled by having its field fully excited or unexcited as required by the load and the generator's ability to recover and hold terminal voltage. A generator so controlled undergoes transitions between load and no load at a rate determined by the generator, the voltage regulator and the load. A disadvantage of this on-off type of regulator is low frequency of correction causing high line ripple. This ripple tends to produce a high level of radio noise and poor quality power.

Such on-off control also suffers disadvantages when controlling a parallel system of DC generators supplying current to a common load. Normally, the on or off state of the field excitation means is determined by the output level of the generator and the amount of load it is carrying, In such a case, the localgenerator calls for changes in excitation at a rate dependent upon its own inherent speed of response to the excitation. An operational frequency of the on-off excitation means is thus established that is related to the natural resonant response frequency of the generator. This phenomenon causes a problem when two or more generators so controlled are paralleled to the same load because normal differences in their respective speeds of response to excitation cause a load unbalance between the generators and set up oscillation of load between them.

Another operational problem encountered with generators controlled by such on-ofi field control centers on their drive shafts. A generator, whether operating alone or in parallel with other generators is driven by a motor or other prime mover. Torque is transmitted from the prime mover to the generator through a shaft normally able, if not especially constructed to absorb and store temporarily a certain amount of torque to enable it to compensate for sharp load transitions imposed upon the prime mover by the generator. The size of the mechanical load presented to the prime mover and shaft varies with the amount of electrical energy being produced by the generator. The transitions between load an no load present in a generator having its output voltage regulated by alternate energization and deenergization of the field are transmitted by the shaft to the prime mover as mechanical load variations. Each load transition is large, since the generator is switching essentially from no load to full load and vice versa.

These mechanical load transitions may be smoothed by being temporarily stored in the shaft between the generator and the prime mover. If the transitions occur at a sufficiently low rate each will be smoothed and dissipated before the next occurs. If they occur at a somewhat higher rate the shaft may be stressed in one direction and then the other continually at such a rate that the shaft is fatigued and fails after only about 10 percent of its normal life. It has been found that voltage regulators that operate by alternately energizing and deenergizing the generator field at a variable rate determined by the generator, the load and the voltage regulator tend to load and unload the generator at a rate that stresses the shaft continually and may be so close to a resonant frequency of the shaft that the load variations will be reinforced by the shaft itself. This phenomenon has been found to occur both in single generator systems and in parallel generator systems.

SUMMARY OF THE INVENTION A preferred embodiment of this invention comprises cooperating and interrelated circuit means to control the operation of a local DC generator acting alone and in a system of paralleled generators in accordance with predetermined operating conditions and in an appropriate and predetermined manner under fault conditions which may exist or occur in the system.

The circuit means includes a field excitation control circuit of the on-off type in which a solid-state switch is controlled to feed current pulses at a relatively high frequency and of varying widths to the field winding of the local generator in a pulse width modulation arrangement. The pulsing switch is controlled by an integrated circuit linear operational amplifier operated as a bistable device. The bistable device is provided with a differential input and is responsive to sensed and algebraically combined indications of generator output voltage, a relatively high frequency pulsing signal, and a load equalization signal and the relationship of this combined signal to a reference level. The load equalization signal is provided by another integrated circuit linear operational amplifier operated as a modified differential amplifier and responsive to the sensed voltage difference resulting from any unequal sharing of the load by the local generator and other paralleled generators of the system. A unijunction oscillator provides the pulsing signal at intervals substantially shorter, preferably about one-tenth as long, than the response time of the generator and mechanical driving system to variations in load. The pulsing signal is a periodic sawtooth signal which serves to pulse the excitation switch once each cycle with a pulse of current whose duration depends upon the generator output voltage signal and the load equalization signal. The excitation current pulses are supplied to the field winding of the local generator at the rate determined by the unijunction oscillator. Although the excitation is of the on-off type, the rate of excitation is forced to be so much faster than the normal response time of conventional on-off-type regulators that the generator responds as though it were receiving constant proportional excitation and enjoying all the benefits associated therewith. In particular the high rate of energization and deenergization of the generator field produces load variations at much too high a rate to be followed by the generator shaft and associated mechanical system. Consequently, mechanical load variations are so attenuated as not to unduly stress the generator shaft. Thus, the field excitation control circuit of this invention combines the advantages of a proportional controller and of an onoff-type controller to solve the problems and remedy the disadvantages inherent in conventional applications of each of those control methods.

The automatic line contactor control circuit comprises the combination of a first differential input integrated circuit linear operational amplifier operated as a bistable device for providing on-off control of the main contactor connecting the generator to the load bus and a second differential input integrated circuit linear operational amplifier connected as an integrator to perform the dual functions of sensing the voltage between main load bus and local generator before the main conductor is closed and sensing reverse current flow into the local generator after the main contactor is closed. When the main contactor is open, the differential input of the first operational amplifier senses the voltage difference between the main lbad bus and the local generator. When the voltage of the local generator equals or exceeds voltage of the main bus, the second and integrating operational amplifier begins to integrate; and, if the condition is maintained, the integrated output causes the energization of the main contactor coil and thereby connects the local generator to the load bus. When the main contactor is closed, the first differential input operational amplifier senses any voltage difi'erence between its terminals caused by reverse current flow to that local generator. When such a condition is detected, the second and integrating operational amplifier begins to integrate the signal; and, if the condition maintains itself long enough, the integrated signal will cause the main contactor to be deenergized, disconnecting the local generator from the main load bus and from the equalizer bus. The automatic line contactor control circuit of this invention combines a differential input integrator and onoff-type control-elements to control the main contactor in an advantageous, efficient, and reliable manner and without special relays. In addition, the circuit performs both the function of differential voltage sensing and the function of reverse current sensing.

The field relay trip control circuit disclosed senses fault conditions in the system of which the local generator is an element, and controls connection of the generator to the system in accordance with a predetermined desired course of action. A preferred embodiment of the field relay trip control circuit includes solid-state switches for disconnecting the generator from the line, means for detecting local ground faults, a first differential input integrating operational amplifier for sensing start ground faults, and a second differential input integrating operational amplifier responsive to sensed conditions of overvoltage, overexcitation and a voltage regulator failure, and a start ground fault for disconnecting the local generator from the parallel system when an undesirable condition continues for a predetermined length of time. Local ground faults developing after the local generator is connected to the main load bus are detected by a current transformer sensing load current at a point as near as possible to the local generator and another sensing load current at a point as near as possible to the main load bus. Any difference in load current detected by the two current transformers actuates means to disconnect the local generator from the system and the load fault condition from the system.

In addition to the individual functions of each circuit described above, the separately described circuits interact and cooperate to complement and supplement the principal functions of each circuit. Means are included, for example, to implement predetermined fail-safe alternatives in the event the control circuitry or some portion of it ceases to function in the normal manner.

Throughout the embodiment of this invention described below, operational amplifiers, and particularly differential input integrating amplifiers, are used as sensing and control elements in conjunction with discrete solid-state switching elements to gain the advantages and substantially avoid the disadvantages of the electromechanical proportional-type control devices and the solid-state on-ofi' devices. By the use of the differential input integrating amplifiers, proportional weighting is achieved through signal integration without the use of heavy and expensive special relays. These and other features and advantages of this invention are disclosed and described below.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I illustrates in combined schematic and diagrammatic form a parallel DC generator system and the control and protection system of this invention and the overall relationship between them.

FIG. 2 is a schematic representation of the field excitation control section and paralleling section of the system;

FIGS. 3A through 3F are diagrams of input and output voltage waveforms as they occur under different operating conditions in the field excitation control circuit of FIG. 2:

FIG. 4 is a diagram of a prime mover drivingly connected through a torsionally flexible shaft to a generator, the diagram being useful in explaining the stresses imposed upon the mechanical system of a generator by alternately energizing and deenergizing the generator field at a low rate.

FIG. 5 is a schematic diagram of the automatic line contactor control section of the system; and

FIG. 6 is a schematic diagram of the field relay trip control section of the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred form and embodiment of this invention and its relationship to a typical DC generator and a parallel DC generator system is depicted in FIG. 1. Broken outline boxes 5, 6 and 7 represent a plurality of identical local DC generators; and, broken outline boxes 8, 9 and 10, respectively, represent identical control and protection panels embodying this invention and associated with the generators. Local generators S, 6 and 7 are each connected in parallel with each other to main bus L and, through their associated control and protection panels, to equalizer bus E. Since the generators and their associated control and protection panels are thesame, only local generator 5 and its associated panel 8 are shown in schematic and diagrammatic detail. It will be understood that any number of such generators and control and protection panels may be connected together in the manner shown to main bus L and equalizer bus E.

Local generator 5 includes an armature 11, an interpole winding 12, and a shunt field winding 13. Local generator 5 is provided with generator terminals 14 and 15, shunt field terminal l6 and neutral terminal 17, the latter being grounded.

Armature 11 is connected by means of line 18 to one terminal of main contactor 19. The other terminal of contactor 19 is connected to load bus L. Main contactor 19 is actuated by coil 21. In a typical installation, generator 5 may be located a significant distance from main contactor 19.

A current transformer indicated generally at 22 is inductively associated with line 18 as near as practicable to main contactor 19. Another current transformer indicated generally at 23 is connected as near as possible to the neutral terminal side of local generator 5. The secondary windings of the two current transformers 22 and 23 are connected in series opposing and to ground fault sensing circuitry of the field trip control circuit as described more fully below. Other local DC generators 6 and 7 are connected in the same fashion to load bus L and thereby in parallel with local generator 5.

In addition to main contactor 19, there are three switching relays associated with generator 5 and which comprise conventional elements of the control and protection system included generally within broken outline box 8. They are the field relay 26, equalizer relay 27 and start fault relay 28. Field relay 26 is a latching relay having two separate coils, a trip coil and a reset coil 30 and 31, respectively. When field relay 26 is operated, it connects shunt field 13 to its source of excitation and also provides a means for energizing the actuating coils of main contactor l9 and equalizer relay 27. When field relay 26 is tripped, local generator 5 is deenergized and disconnected from the parallel system.

Equalizer relay 27 has an actuating coil 32 connected to be energized with main contactor actuating coil 21. Equalizer relay 27 connects terminal of local generator 5 through equalizer bus E to the corresponding armature terminals of other generators with which generator 5 is connected in parallel. The equalizer bus E interconnection provides means to determine the relative amount of current supplied by the local generator 5 and the other generators 6 and 7 connected to the equalizer bus. Generator terminal 15-of local generator 5 is a convenient point for sensing the amount of load supplied by the generator as will be more fully disclosed hereinafter.

Start fault sensing relay 28 provides a path for detecting a local ground fault before the local generator 5 is connected to main bus L and operates as described more fully below.

The system for controlling and protecting a typical local generator and associated switching relays such as generator 5 and relays 26, 27 and 28 described above must perform a number of control functions. The control system must determine, for example, the appropriate time to connect the local generator to the main bus. To do so, the controls must determine that there is no appreciable voltage difference between the main bus and the local generator. Once the generator is connected, it must be determined that the generator is supplying current to the load and not taking current from the load bus. It must also be determined that the local generator is supplying its share of the current supplied by all the generators on the line. The output voltage level of the local generator must also be maintained at a predetermined desired level. The control panel must sense fault conditions such as a local ground fault, i.e., a ground fault on the generator side of its associated main contactor connecting it to the main bus. Such a fault causes the generator to supply a large amount of current to the fault instead of to the main bus. Likewise, a remote ground fault on the main bus causing the entire generator system to supply current to that ground fault instead of to the proper load must be sensed and the appropriate action taken. Numerous other conditions and faults may arise which require automatic sensing and control action from the control circuits.

The functions described above and other functions are performed in a reliable and advantageous manner by the preferred form of control and protection panel 8 described below and comprising three principal control sections which interact together and with local generator 5 and its associated switching relays. These sections are the automatic contactor control 34, the field excitation control 35, and the field relay control 36. In FIG. 1, these control sections are shown in block form within control and protection panel 8. They are shown in detail in other figures of the drawings and each is fully described below in connection with them.

All power to the principal sections of the DC generator control panel is supplied by its associated local DC generator.

FIELD EXCITATION CONTROL CIRCUIT FIG. 2 is a schematic diagram of a preferred form of field excitation control section 35 in detail and in connection with local generator 5 shown in FIG. 1 and described above. Power for this section, as well as all sections of the control and protection panel, is supplied by the associated local generator. In FIG. 2, a positive bus 38 is connected to generator terminal 14 and a grounded bus 39, to neutral terminal 17. Control section 35 comprises an integrated circuit linear operational amplifier 40 connected as a differential switch amplifier and having a summing input 41, a reference input 42, and an output 43. Broadly, operational amplifier 40 controls the conductive state of a transistor switch controlling the connection of shunt field 13 of generator 5 across generator terminals 14 and 15 and thus the field excitation of the generator. Operational amplifier 40 responds to voltages applied to its inverting and noninverting inputs as described more fully below.

Operating power is supplied to operational amplifier 40 in a conventional manner by connection to positive bus 38 and grounded bus 39. The gain and biasing of operational amplifier 40 are such that it operates as a bistable device providing a substantially square wave output. Summing input 41 is an inverting input with respect to output 43. Reference input 42 is a noninverting input which is connected to a source of reference voltage E as shown. Operational amplifier 40 is triggered to one of the stable states providing a more positive voltage at output 43 whenever the voltage at summing input 41 is less than the voltage E, at reference input 42. When the voltage at summing input 41 is greater than the reference voltage E, at input 42, amplifier 40 assumes its less positive output state.

Output 43 of operational amplifier 40 is connected through the series combination of Zener diode 44 and resistor 45 to thebase 46 of PNP-transistor 48. Base 46 is also connected through resistor 47 to positive bus 38. Emitter 49 and collector 50 of transistor 48 are connected to positive bus 38 and grounded bus 39, respectively, the latter through resistor 51.

Shunt field winding 13 of local generator 5 is excited from generator terminal 14 through the collector-emitter circuit of PNP-transistor 53 of the Darlington-connected pair 52 and 53. The collector 55 of transistor 53 is connected to contact 56 of field relay 26 and thereby to shunt field tenninal 16.

The network described above and connected to output 43 of operational amplifier including transistor 48 and Darlington-connected pair of transistors 52 and 53 function to connect and disconnect shunt field 13 and generator terminal 14 of local generator in accordance with the stable state assumed by operational amplifier 40. When the output of operational amplifier 40 is more positive, conduction by Zener diode 44 is blocked and transistor 48 is held nonconducting by the resulting positive voltage level at its base 46. When transistor 48 is blocked, Darlington pair 52 and 53 conducts and shunt field I3 is excited from generator terminal 14 of its local generator 5. When the generator of operational amplifier 40 local less positive, Zener diode 44 conducts, providing a lower voltage level at base 46 and permitting transistor 48 to conduct. When transistor 48 conducts, Darlington pair 52 and 53 is held noncondueting and shunt field I3 is disconnected from generator terminal 14. A return diode 57 is connected in parallel with shunt field 13 as shown to provide a path for an induced current to circulate when excitation of the shunt field is removed.

The voltage at summing input 41 is the sum of three voltages introduced through resistors 58, 59 and 60 as shown in FIG. 2. The sum of the voltages at these three inputs determines whether or not the voltage level at summing input 41 is lower or higher than the level E at reference input 42. The voltages applied to summing input 41 through resistors 58 and 59 may be considered primarily as voltage regulator signals while the input through resistor 60 is a load regulation signal.

The voltage coming to summing input 41 through resistor 59 is obtained by sampling the local generator voltage directly through a voltage divider arrangement of resistors 61, 62, and 63 connected across positive bus 38 and grounded bus 39.

The voltage supplied to summing input 41 through resistor 58 has a sawtooth waveform delivered from emitter 64 of unijunction transistor 66. Unijunction transistor 66, resistor 67 and capacitor 69 comprise an oscillator providing a sawtooth waveform output. Unijunction transistor 66 is a negative resistance device in that over a portion of its operating range when the emitter to base l; current increases, the voltage from to base 11, decreases. In the circuit shown, capacitor 69 charges through resistor 67 from bus 38 until the negative resistance region of unijunction transistor 66 is reached. Base b is connected to a regulated voltage supply network comprised of resistor 70 and voltage reference diode 71. At this point, unijunction transistor 66 conducts and the voltage from emitter to base b, drops sharply. Capacitor 69 is then discharged through unijunction transistor 66. The sawtooth waveform provided by the charge and discharge of capacitor 69 is coupled to summing input 41 through voltage division network comprised of resistors 68a and 68b, and through capacitor 72, thereby removing the DC level. The frequency of the oscillator output depends upon the time constant of the RC combination, the voltage level of the bus, and is preferably relatively high, e.g., about I kilocycle.

The signals applied to summing input 41 from resistors 58 and 59 consists of a voltage proportional to the output of generator 5 on which is superimposed the zero-referenced sawtooth waveform from the unijunction transistor 66 oscillator. The negative portions of the sawtooth component of this combined signal cause the voltage level at summing input 41 to fall below the level if reference input 42, triggering operational amplifier 40 to its higher voltage output stable state. This turns on transistors 52 and 53 and injects a pulse of excitation current into shunt field winding 13 of local generator 5. Under normal operation conditions, the output voltage of local generator 5 will be such that the generator output proportional voltage at summing input 41 will be slightly greater due to the superimposition of the sawtooth waveform than the voltage at the reference input 42.

The operation of the field excitation control element of the system under three different operating conditions is illustrated in FIG. 3 of the drawings. FIG. 3A shows voltage waveforms of the summing input 41 and the reference input 42 and FIG. 3B shows the corresponding voltage waveform at output 43 of operational amplifier 40. In FIG. 3A, line 73 represents the voltage level at reference input 42. Line 74 represents the output-proportional DC voltage at summing input 41 on which is superimposed AC sawtooth signal 75 from unijunction transistor 66 oscillator. The output voltage of the local generator is substantially greater than the reference level. Whenever sawtooth signal 75 drops below the reference level 73, operational amplifier 40 causes Darlington pair 52 and 53 to conduct; and, when sawtooth signal 75 rises above reference level 73, operational amplifier 40 causes Darlington pair 52 and 53 to turn off. FIG. 3B illustrates the waveform 76 at the output of operational amplifier 40 which corresponds to the inputs illustrated in FIG. 3A. Field winding 13 receives excitation current during the positive portions of waveform 76.

FIGS. 3C and 3D illustrate waveforms corresponding to those in FIGS. 3A and 3B but when the output voltage of the local generator is almost equal to the desired or reference level. The corresponding output wavefonn of FIG. 30 shows that shunt field winding 13 receives longer current pulses, thereby increasing the excitation to local generator 5.

As the voltage level represented by line 74 falls below the reference level depicted by line 73, the duty cycle of excitation pulses exceeds 50 percent and approaches the condition of constant excitation. The input and output voltage waveforms corresponding to this operational condition are shown in FIGS. 3E and 3F, respectively.

The duty cycle and level of excitation for a given generator may also be controlled by changing the slope and amplitude of sawtooth signal 75.

Through the use of pulse width modulated control of the excitation means, voltage regulation is accomplished in an im proved and in a smooth and efficient manner with a minimum of overshoot and oscillation or hunting of the output voltage of the local generator.

In previous on-ofi-type voltage regulators, the relative duration of the on and off states of the field excitation means was determined solely by the output level of the generator relative to the desired level. In such a case, the local generator would call for excitation or no excitation at its inherent speed of response to the excitation. An on-ofi' frequency of the excitation means was thus established corresponding to the natural resonant response frequency of the local generator. This frequency was normally much lower than the frequency of the sawtooth function signal described above in connection with this invention and, therefore, regulation of the output voltage was not as smooth as that possible with controls embodying this invention. This operational characteristic is also a problem when generators are used in parallel. When two or more generators controlled in the prior art fashion described above are paralleled to the same load, their normally different inherent speeds of response to excitation cause a load unbalance which oscillates and beats between the generators. Each paralleled generator effectively overshoots its varying share of the load in both directions. A more serious problem occurs in the mechanical portions of the generator driving system as will be explained below.

In FIG. 4 a prime mover 77 is shown drivingly connected to a generator G through a shaft 80. Prime mover 77 may be, for example, an aircraft engine. Generator G has a stator portion 81 including a magnetic field winding 82 and a rotor portion 84 that includes the customary armature winding. The rotor is mounted on shaft and supported in a suitable manner. A vibration damper 85 is connected to shaft 80 adjacent the prime mover to damp vibrations transmitted from prime mover 77 to shaft 80 and to generator G. Shaft 80 may be of any construction that allows it to be resilient in torsion so that it is able to temporarily store energy transmitted between prime mover 77 and generator G and to smooth out load fluctuations. A more detailed description of such a shaft, or torque rod, and its connection between a prime mover and a generator is given in U.S. Pat. No. 2,658,36l to Kalikow.

The output voltage of generator G is regulated by an on-offtype voltage regulator 87 which alternately energizes and deenergizes generator field 82. The output of generator G is connected to a bus 89. An additional DC power source 90, such as a battery or another DC generator, may also be connected to bus 89.

Energization and deenergization of the field of generator G causes the generator to undergo transitions from essentially full load to no load and vice versa. These transitions are presented to shaft 80 and prime mover 77 as mechanical load variations for which the prime mover will attempt to compensat'e. To smooth out these variations shaft 80 twists and untwists or reflects angularly to store temporarily and then release the load. The amount of twisting imposed upon the shaft is substantial since the load variations are essentially from no load to full load.

As discussed above, if the transitions occur at a sufi'iciently low rate each will be smoothed and dissipated before the next occurs. It has been found, however, that in generators having their output voltage regulated by on-off-type regulators an onoff rate of regulation is very likely to be established that causes variations in mechanical load on the shaft to occur at a rate that is deleterious to the life and integrity of the shaft. This rate is such that the shaft is continually in torsion in one direction and then the other, and the rate is sufficiently low that the shaft can follow each load variation to its full amplitude. In effect, the shaft is undergoing continual torsional stress at a resonant frequency. As a result, the shaft and associated mechanical components are fatigued and may fail after only 10 percent of their normal lives. The vibration damper 85 when provided may absorb the load transitions for a short time. It soon fails, however, and the shaft is left to bear the entire stress of the load variations with the result just described. Fatiguing of typical shafts under these conditions has been observed for on-off voltage regulation rates from about 30 cycles per second to approximately 100 cycles per second.

This phenomenon has been observed in both single generators and parallel generator systems. In single generator systems a destructive rate of field energization and deenergization is found to occur most often when a battery is connected to a generator output to be charged thereby. The battery is thought to act as a capacitor and dominate the rate of voltage regulation. In parallel generator systems the destructive rate is apparently brought about by the interplay of parallel generators which overshoot their share of the load in both directions as described above.

As a solution to this problem it has been discovered that if the field is energized and deenergized, not at a variable rate determined by the generator system and its load, but positively and periodically at a rate substantially higher than the generator system and load would choose, the mechanical system particularly the shaft, is not able to follow the individual periodic corrections. As a result the amplitude of the mechanical load variation is greatly attenuated and the shaft and associated mechanical system are not unduly stressed. The interval between successive energization signals should be substantially shorter, preferably about one-tenth as long, than the response time of the mechanical system of the generator to applied or removed field excitation. The voltage regulator circuit shown in FIG. 2 is preferred for periodically energizing and deenergizing the generator field at intervals such that the mechanical system of the generator is incapable of following the load variations.

The response time of the mechanical system of a generator varies with the mass of the rotating parts. The mechanical systems of lighter weight generators, including the driving shaft, are capable of following load variations that occur at higher rates than can be followed by more massive generators. Response times for different generators have been determined by observing the amplitude of the torsional strain imposed on the driving shafts for difierent rates of field energization and deenergization. The amplitude of the strain decreases as the on-off rate of field energization is increased. As noted above fatiguing of driving shafts has been observed for on-off regulation rates from about 30 cycles per second to about I00 cycles per second for particular generators. The periods of these signals, from about 33 milliseconds to about 10 milliseconds, are an indication of the response times of the generators mechanical systems to torsional inputs produced by applied or removed field excitation.

The amplitude of torsional strain imposed upon the shafts was reduced to a quite acceptable value when the on-off regulation rates were increased by a factor of about 10 above those at which fatiguing occurred. Accordingly, it is believed that a forced on-off regulation rate of about 10 times the rate at which the shaft can follow the load variations is desirable to prevent fatiguing of the shaft. Or, in other words, it is desirable that the interval between field energization pulses be about 10 times shorter than the response time of the generator mechanical system, as determined by observing the amplitude of torsional strain on the shaft, to applied or removed field excitation.

In the system of controls embodying this invention, the problems noted above are solved by having the generators excited by pulses of current at a rate much higher than the response time of the generators mechanical system. The rate of excitation is constant and substantially the same for all generators. At this higher rate, the generators in a parallel system have a substantially reduced time to overshoot and the electromagnetic and mechanical systems of the local generator cannot overrespond to it. Also, as previously described, diode 57 tends to maintain a smooth flow of current through field winding 13 by providing a path for the current induced in field winding 13 during the off time of the excitation means. More importantly, the shaft associated with either a single or paralleled generator is incapable of following the higher on-off rate and so is not driven to early failure.

The third input through resistor 60 to summing input 41 of operational amplifier 40 is for the purpose of load equalization between local generator 5 and the other generators on the line. This input is provided by an operational amplifier 97 which detects differences, if any, in the amount of load being supplied by local generator 5 and the other generators on the line. Operational amplifier 97 is connected as a modified differential amplifier having feedback paths through resistor 98 and its parallel-connected bypass capacitor to its inverting input 99, and through resistor 112 and its parallel-connected bypass capacitor to its noninverting input 101 from reference voltage source E Operational amplifier 97 also has an inverting input resistor 100 and a noninverting input 101 and input resistor 102. Inputs 99 and 101 are tied through their respective input resistors to the opposite ends of a resistor 103 inserted in series between generator terminal 15 of local generator 5 and equalizer bus E. Connection to equalizer bus E is made through contact 104 of equalizer relay 27.

The relative amounts of current being supplied by local generator 5 and the other paralleled generators are determined by comparing the voltage appearing at terminal 15 with the voltage appearing at corresponding terminals of the other paralleled generators. As increasing current is supplied by local generator 5, for example, to load bus L, the voltage at generator terminal 15 becomes increasingly negative with respect to neutral terminal 17. By connecting the resistor 103 between generator terminal 15 and the equalizer bus E, the magnitude and polarity of any difference in voltage between terminal 15 and the equalizer bus is sensed across resistor 103. Resistor 113 provides decoupling of Zener diode 105 from terminal 15 of the generator.

The voltage level at terminal 15 of the generator, however, is below the common mode input range of operational amplifier 97. In order to raise the common voltage level at resistor 103 to the voltage level required by the input of operational amplifier 97, Zener diodes 105 and 106 are connected in series with resistors I07 and 108, respectively, and to positive bus 38. Zener diodes 105 and 106 have identical values of Zener voltage so that the voltage across resistor 103 is transferred to the input resistors 100 and 102 of operational amplifier 9 7 with its relative magnitude and polarity preserved, but transformed in level to the operating range of operational amplifier 97. The output voltage of operational amplifier 97 appearing at 109 and through filter network resistor 114 and capacitor 155 comprises the third signal for the summing input of operational amplifier 40 described above. This filtered output is connected by line 111 to input resistor 60 at summing input 41. Output at line 111 is also connected through equalizer relay contact 110 to A-A in FIG. described below in connection with the protection circuit.

In operation, if local generator 5 provides more than its share of current to load bus'L, the voltage at generator terminal will be more negative than the voltage at the equalizer bus E. This voltage difference will be developed across resistor 103 and transferred to the input resistors 100 and 102 of operationalamplifier 97. The voltage at inverting input 99 will be negative with respect to the voltage at noninverting input 101, causing output voltage at 109 of operational amplifier 97 to go positive. The preferable ratio of values of resistors 98 and 100 is such that the gain of operational amplifier 97 is nominally 50. The positive signal appearing at output 109 is then transmitted through resistor input 60 to summing input 41 of operational amplifier 40 where it is algebraically combined with the DC input that is proportional to local generator 5 output and with the sawtooth function signal described above. The DC voltage level at summing input 41 thus tends to become more positive with respect to the voltage at reference input 42, thereby decreasing the width of each pulse of excitation current transmitted to field winding 13 in the manner described above in connection with FIGS. 3A and 3F. The excitation to field winding 13 is thus decreased and the load carried by local generator 5 is reduced to the level of load carried by the other parallel generators.

If the voltage at terminal 15 of local generator 5 is positive with respect to the voltage at equalizer bus E, the voltage developed across resistor 103 makes inverting input 99 of operational amplifier 97 positive with respect to noninverting input 101. As a result, output 109 of operational amplifier 97 goes negative and causes the voltage level at summing input 41 of operational amplifier 40 to go more negative with respect to reference input 42. As described above, such a change in the relationship of the levels of the DC input and the reference input increases the width of the excitation pulses. Field winding 13 receives longer pulses of current at the 1000 cycle per second rate, thereby increasing the level of field excitation to local generator 5. Local generator 5 will thus tend to contribute more current to load bus L.

From the foregoing, it is apparent that operational amplifier 97 and its associated circuitry attempt to sum to zero the voltage difference between terminal 15 of local generator 5 and the equalizer bus E.

Another feature of this invention is embodied in the field excitation control circuit described above and shown in FIG. 2. This feature includes means to insure that local generator 5 under control of the field excitation circuit is fully and continuously excited to the extent of its output in the event its output voltage is pulled down by a fault or other cause to and below the level required to operate the control circuitry. As long as the output voltage of the generator remains high enough to operate the control circuitry, the field excitation is controlled by the circuit in the manner described above. Such an operating characteristic is normally preferred in aircraft DC generator power systems as well as other systems.

Referring to FIG. 2, this failure condition operating characteristic is provided by Zener diode 44 connected in series with resistors 45 and 47 between the output of operational amplifier 40 and the generator output as it appears at positive bus 38. As described above, the conducting state of Zener diode 44 controls the excitation of field 13 of the generator through connection of base 46 of transistor 48 to the junction of resistors 45 and 47. When Zener diode 44 conducts, field 13 is deenergized; when Zener diode 44 blocks, field 13 is excited. Zener diode 44 conducts when the voltage applied to its cathode from the output of the generator through positive bus 38 and resistors 45 and 47 exceeds the voltage applied to its anode by operational amplifier 40 by an amount greater than the Zener voltage of the diode.

The Zener diode is selected so that, under normal operating conditions with the generator output voltage maintained at or near the reference voltage E and no substantial faults present tending to pull down the output voltage, conduction of the diode is controlled by the stable state of operational amplifier 40. When the output of operational amplifier 40 is less positive, the difierence between it and the substantially fixed generator output voltage exceeds the Zener voltage of diode 44 and it conducts. When operational amplifier 40 is in its other stable state providing a more positive output, the applied voltage difference across Zener diode 44 is less than its Zener voltage and it blocks.

Under large fault conditions which drag the generator output voltage down below the level required to operate operational amplifier 40 and the control circuitry, operational amplifier 40 provides a less positive output which normally calls for deenergization of the generator to local or the opposite of the desired function under the fault conditions. As the generator output voltage is brought down by the fault, however, the difference between it and even the operational amplifiers less positive output voltage falls below the Zener voltage of Zener diode 44 and field 13 of the generator is now continuously excited to load bus extent possible by the declining or reduced operating characteristic for a large fault current condition is provided by this invention as embodied, for example, in the circuit of FIG. 2.

AUTOMATIC LINE CONTACT OR CONTROL CIRCUIT FIG. 5 illustrates in schematic form a preferred embodiment of the automatic line contactor control circuit of this invention connected in operating relationship to local generator 5. Broadly, the function of the automatic line contactor control circuit is to control the flow of current to line contactor coil 21 and equalizer relay coil 32, thereby controlling the connection of local generator 5 to load bus L and to the equalizer bus E. Local generator 5 must be connected to or disconnected from load bus L and equalizer bus E at various times and for various reasons as will be more fully discussed below.

In FIG. 5, the automatic line contactor control circuit comprises an operational amplifier 116 connected as an integrator having a feedback capacitor 117, inverting input 118 and resistor 119, "and noninverting input 120 and resistor 121. Inverting input 118 is shown monitoring the output voltage of generator 5 through voltage-dividing resistors 122 and 124 connected in series between the point of regulation, as shown, and neutral terminal 17. Noninverting input 200 monitors the voltage between load bus L and generator terminal 15 of local generator 5 through voltage-dividing resistors 125, 126 and 127.

During buildup of local generator 5, main actuator 19 and equalizer relay 27 are open, isolating local generator 5 from load bus L and equalizer bus E. Before main contactor 19 is allowed to close connecting local generator 5 to load bus L, the automatic line contactor control circuit must detennine that the output voltage of local generator 5 is either greater than, or only very slightly less than, the voltage on load bus L so that local generator 5 can supply current to load bus L instead of taking current from it. This detennination is accomplished and indicated by operational amplifier 96 as described below.

When main contactor 19 is open and local generator 5 is not supplying the load, the generator delivers only enough current to energize its associated control circuitry. This is a relatively small amount of current. As a consequence, the voltage at terminal 15 of local generator 5 will then be very close to ground potential. In such case, noninverting input effectively monitors the voltage level above ground of load bus L. Inverting input 98 monitors the output voltage of local generator 5.

A voltage proportional to the difference between local generator output and load bus L appears then between the inverting and noninverting terminals 118 and 120, respectively, of operational amplifier 116.

When the voltage at inverting input 118 becomes positive or only very slightly negative with respect to the voltage appearing at noninverting input 120, operational amplifier 116 begins to integrate in the negative direction and continues to integrate in the negative direction as long as this input voltage relationship exists. The slope of the output signal is determined by the time constant of feedback capacitor 117 and inverting input resistor 119 and filter network resistor 111 and capacitor 163 and the magnitude of the voltage difference at the inputs. g

The output 128 of operational amplifier 116 is coupled to the summing and inverting input 129 of operational amplifier 130 through resistors 131 and 123. Resistor 154 and diode 155 are returned to terminal 180 of amplifier 181 in FIG. 5. Operational amplifier 130 is connected to operate as a bistable device. Noninverting input resistor 133 is connected to a voltage reference 134 through resistor 135 and to output 136 of operational amplifier 130 through resistor 137 in the feedback connection. Feedback resistor 137 functions to supply positive feedback to noninverting input 132, thereby enhancing the step function output response of operational amplifier 130. Output 136 of operational amplifier 130 is connected to the base of transistor 141 through the series combination of resistor 138 and Zener diode 139. Transistor 141 responds to the conductive state of Zener diode 139 in the same fashion as transistor 48 in the field excitation control circuit described above in connection with FIG. 2. The base 140 of transistor 141 is also connected to load bus L through a bleeder resistor 137a. The collector of transistor 141 is tied to ground through the series combination of resistor 143 and Zener diode 144. A bypass to ground around Zener diode 144 is provided by NPN-transistor 147. The collectors of transistors 145 and 146 are connected through resistor 148 to the base of transistor 147 for controlling its conductive state. Transistor 141 is connected to and controls the conductive state of a Darlingtonconnected pair of PNP-transistors 145 and 146.

Positive bus 38 is connected to or disconnected from ac tuating coil 21 of main contactor 19 and through it to ground by the switching operation of PNP-transistor 146 and contact 149 of field relay 26, and manual control switch 181. When transistor 146 conducts and contact 149 and switch 181 are closed, actuating coil 21 of main contactor 19 is energized. When either or both transistor 146 and contact 149 are open, actuating coil 21 is deenergized. Opening of manual switch 201 will also result in the deenergization of actuating coil 21 of the line contactor. Actuating coil 32 of equalizer relay 27 is connected directly in parallel with actuating coil 21 and with a bypass diode as shown. Actuating coil 151 of start fault relay 28 and its associated bypass diode 203 are also connected in parallel through a blocking diode 152 which functions with contact 153 of start fault relay 28 to lock in relay 28 from positive bus 38 without energizing coils 32 and 21.

In operation, when the output voltage of operational amplifier 116 causes the voltage level at the inverting input 129 of operational amplifier 130 to become more negative than the voltage level at noninverting input 132, the output of operational amplifier 130 will be a positive-going step function blocking Zener diode 139 which, in turn, turns off transistor 141. When transistor 141 turns off, the voltage at the collector of transistor 141 becomes less positive, thereby turning on Darlington pair 145 and 146 and allowing the base current of transistor 145 to flow to ground through resistor 143 and Zener diode 44. When Darlington pair 145 and 146 is caused to conduct, the voltage level appearing at its collectors causes transistor 147 to conduct, thereby short-circuiting Zener diode 144 and cutting it off. Transistor 147 also provides a path for the base current of transistor 145, thereby tending to hold the Darlington pair conducting. When Darlington pair 145 and 146 is conducting, current flows through contact 149 of field relay 26 to energize coil 21 of line contactor 19, coil 32 of equalizer relay 27 and coil 151 of start fault relay 28 which locks in through its own contact 15153. Main contactor 19 and equalizer relay 27 are thus caused to connect local generator 5 to load bus L and to the equalizer bus B when the generator output voltage at inverting input 118 of operational amplifier 116 equals or is only slightly less negative than the load bus L voltage sensed by noninverting input 120.

Once main contactor 19 is closed and local generator 5 connected to load bus L, operational amplifier 116 and its associated circuitry perform the function of detecting reverse current through local generator 5. Ideally, when local generator 5 is connected to load bus L, there is no voltage difference existing between load bus L and output terminal 14. Under normal circumstances, the field excitation control circuit equalizes any difference in the load shared between local generator 5 and the other parallel generators on the line as explained above. If local generator 5 does become a load on the load bus L, however, it is desirable to take it off the line. This function is accomplished in the following manner.

Current supplied or taken by local generator 5 causes a corresponding voltage drop across interpole winding 12. The magnitude and direction of current supplied or taken by local generator 5 may thus be determined by monitoring the magnitude and polarity of the voltage level appearing at terminal 15 of local generator 5. As described above, when operational amplifier 116 is operating to sense the voltage difference between load bus L and the output of local generator 5 for automatic line contactor control, the voltage appearing at terminal 15 is essentially ground potential. In the reverse current sensing mode, the voltage of the load bus L and of local generator 5 are normally equal; but, should there be reverse current flowing through the interpole winding 12, the voltage appearing at terminal 15 becomes positive with respect to ground, causing an increase in the voltage level at noninverting input of operational amplifier 116 with respect to inverting input 118. The output of operational amplifier 116 will then begin to integrate in the positive direction at a rate determined by the time constant of capacitor 117 and resistor 119, and filter resistor 11] and capacitor 163, and the magnitude of the difierential input signal.

When the voltage at inverting input 129 from the output 128 of operational amplifier 116 becomes more positive than the reference voltage at noninverting input 134 from voltage reference 134, the output of operational amplifier will be a negative-going step function permitting Zener diode 139 to conduct which, in turn, causes transistor 141 to conduct through transistor 147. Darlington pair and 146 is then cut off which, in turn, cuts off transistor 147, causing Zener diode 144 to conduct current from transistor 141. When Darlington pair 145 and 146 stop conducting, actuating coil 21 of main line contactor 19 and actuating coil 32 of equalizer relay 27 will no longer be energized causing main contactor l9 and equalizer relay 27 to open. Contact 149 of field relay 26 remains closed so long as field relay 26 is not tripped as will be discussed more fully below. In this manner, generator 5 is removed from the line and will remain off the line until the desired relationship between the generator outputvoltage and the load bus voltage is reestablished.

Start fault relay 28 is energized through its own contact 153 and will release only when the output voltage of local generator 5 falls below the hold-in current of coil 151. Diode 152 isolates coil 21 of main contactor 19 and coil 32 of equalizer relay 27 from the output of local generator 5. The function of start fault relay 28 is to provide a path for detecting local ground faults before local generator 5 has been connected to load bus L so that such connection can be prevented until the local ground fault is cleared. This operation will be described more fully below.

The summing and inverting input 129 to operational amplifier 130 has an additional input through resistor 154 and diode 155 from the start ground fault sensing portion of the field relay trip control circuit. In the event of a local ground fault prior to closing of main contactor 19, this input operates to keep local generator 5 disconnected from load bus L by preventing main contactor 19 and equalizer relay 27 from closing as will be more fully discussed below. It will be noted that a local ground fault will cause current flow through interpole winding 12 to drive operational amplifier 116 into its less positive state. Such an output, in turn, tends to cause secondstage operational amplifier 130 to make Darlington pair 145 and 146 conduct and close the main contactor 19. Since it is undesirable to put a local generator with a local ground fault in its local network to the line, the additional input signal to inverting input 129 is provided from the ground fault sensing section of the system.

It will be noted that Zener diode 139 functions to provide a fail-safe type of operating characteristic to the automatic line contactor control circuit of FIG. 5 and which is similar to the operating characteristic provided by Zener diode 4 in the field excitation control circuit previously discussed. When local generator 5 is connected to load bus L, a remote ground fault can cause the load bus voltage to fall below the level required for normal operation of the control circuits. In such a case, Zener diode 139 will hold transistor 141 in the off state, thereby causing Darlington pair 145 and 146 to conduct and hold main contactor 19 and equalizer relay 27 energized. Local generator 5 thus is locked onto load bus L to assist the other paralleled generators in burning out the remote ground fault.

Zener diode 144 also provides a fail-safe-type operating characteristic but does so in the opposite case from that in which Zener diode 139 functions. If local generator 5 is tripped ofi' the load bus L because of some fault, Zener diode 144 prevents Darlington-connected transistors 145 and 146 from conducting and connecting local generator 5 to load bus L until the output voltage of local generator 5 is at least equal to the Zener voltage of Zener diode 144. From the foregoing description of the circuit of the line contactor control, it will be apparent that, if the output voltage of local generator 5 is below the Zener voltage of Zener diode 144, transistors 145 and 146 are prevented from conducting for lack of a base current path from Darlington pair 145 and 146 unless transistor 147 is caused to conduct. However, Darlington pair 145 and 146 must conduct in order to turn on transistor 147. Darlington pair 145 and 146 is thus held in the nonconducting state, holding local generator 5 off load bus L until the local generator output voltage at least equals the Zener voltage of Zener diode 144.

Thus, there are at least two operating conditions that must be satisfied before local generator 5 can be connected to load bus L by main contactor 19. First, the output voltage of local generator 5 must be substantially equal to the voltage of load bus L; and, second, the output voltage of local generator 5 must be at least equal to the Zener voltage of Zener diode 144. As a consequence, when a voltage-reducing fault occurs, Zener diodes 139 and 144 hold main contact 19 in its condition at the time of the fault and until the fault is corrected. Lastly, local generator 5 cannot be connected to load bus L if a local ground fault exists provided the ground fault signal is furnished inverting input 129 of second-stage operational amplifier 130 as shown in the circuit drawing.

FIELD RELAY TRIP CONTROL CIRCUIT FIG. 6 illustrates in schematic form a preferred embodiment of the field relay trip control circuit of this invention in its operational relationship to local generator 5. Broadly, the function of this circuit is to trip field relay 26 in the event that a malfunction of some type occurs. Field relay 26 is a latchingtype relay having a reset coil 31 and a trip coil 30. The field relay trip control circuit controls the energization of field relay trip coil 30. When field relay 26 is tripped, shunt field winding 13 of local generator 5 is disconnected from its source of excitation, and main contactor 19, equalizer relay 27 and start fault relay 28 are deenergized, causing local generator 5 to be completely disconnected from the parallel generator system.

in FIG. 6, operational amplifier 156 is connected as an integrator having feedback capacitor 157 and feedback diode 158. Connected to summing and inverting input 159 are summing input resistors 160,161 and 162; and connected to noninverting input 164 through resistor 165 is reference voltage 166. Feedback diode 158 serves as a clamp, allowing operational amplifier 156 to integrate only in the negative direction. When the voltage level at the inverting input 159 of operational amplifier 156 becomes more positive than the reference voltage level at noninverting input 164, the output of operational amplifier 156 begins to integrate in the negative direction. The rate of integration depends upon the time constant of feedback capacitor 157 and summing input resistors 160, 161 and 162 and upon the magnitude of the input voltage. If the summed voltage level at inverting input 159 continues to be more positive than the reference voltage level at noninverting input 164, the output of operational amplifier 156 continues to integrate in the negative direction until the anode voltage applied to Zener diode 168 is lowered enough to reach its Zener voltage'and render Zener diode 168 conduction. Feedback and Zener diodes 158 and 168, respectively, effectively define the limits of integration of operational amplifier 156.

Output 167 of operational amplifier 156 is connected to the anode of Zener diode 168, and the cathode of diode 168 is connected to the cathode of a silicon-controlled rectifier 169. The cathode is also connected through a blocking diode 170 to a voltage reference source 171. The gate of SCR 169 is tied to source 171 on the cathode side of blocking diode 170. The anode of SCR 169 is connected to positive bus 38 through resistors 172 and 173 and trip coil 30 of filed relay 26 as shown. A freewheeling bypass diode 174 is provided for trip coil 30.

When Zener diode 168 conducts, the voltage level at the cathode of SCR 169 falls below the gate voltage, thereby causing SCR 169 to conduct through Zener diode 168. Current through SCR 169 energizes energizes trip coil 30 of field relay 26, opening its contacts 56 and 149 (see FIGS. 2 and 5) and thereby disconnecting field winding 13 from its source of excitation and disconnecting local generator 5 from load bus L and from equalizer bus E. Field relay 26 must be reset by an operator to bring local generator 5 back into nonnal operation.

The signals to which operational amplifier 156 responds are applied through resistors 160, 161 and 162 and algebraically combined at inverting input 159. Resistor 160 is adjustably connected to resistor 177 of a series combination of resistors 176178 comprising a voltage divider connected between positive bus 38 and ground.

The signal applied through resistor 160 is proportional to the output voltage of local generator 5. Potentiometer resistance 177 is set so that, when the output voltage of local generator 5 exceeds a predetermined upper limit, the voltage level at inverting input 159 becomes more positive with respect to the voltage at noninverting input 164, causing the output of operational amplifier 156 to integrate in the negative direction and resulting eventually in local generator 5 being tripped off load bus L. in this manner, operational amplifier 156 senses and provides protection against overvoltages not corrected by the field excitation control circuit.

Variable input resistor 162 is connected through blocking diode 179 to the equalizer section of the field excitation control circuit shown in F IG. 2. Connection is made to output 109 of differential operational amplifier 97 through contact 110 of equalizer relay 27. As described above, a positive signal from operational amplifier 97 indicates that local generator 5 is providing more than its share of current to load bus L. Under normal circumstances, load regulation is accomplished by the field excitation control circuit. However, if, because of some malfunction, operational amplifier 40 of the field excitation control circuit does not regulate the load in the normal manner as described above, the output voltage of operational amplifier 97 continues to rise, causing the voltage level at inverting input 159 of operational amplifier 156 to become more positive and eventually disconnecting local generator from the parallel system. Thus, in the event of uncorrected overexcitation of local generator 5, operational amplifier'156 acts as auxiliary means to protect the local generator.

The signal to inverting input 159 of operational amplifier 156 through resistor 161 is, as shown in FIG. 6, from the output 180 of operational amplifier 181 which is connected as an integrator having feedback capacitor 182 and input resistors 183 and 184 to inverting input 185 and noninverting reference input 186, respectively. lnverting input 185 and input resistor 183 are connected to generator terminal of local generator 5 through Zener diode 187 and contacts 188 of start fault relay 28, and to a source of regulated voltage comprising resistor 189 and Zener diode 191, as shown in FIG. 6. Noninverting input 186 and resistor 184 are connected to ground through Zener diode 192 and to positive bus 38 through resistor 193, all as shown in FIG. 6.

The function of operational amplifier 181 is to sense any local ground fault before local generator 5 is connected to load bus L. When such a local ground fault exists, local generator 5 tends to supply large amounts of current to the ground fault. The large amount of current supplied by local generator 5 will cause the voltage level at generator terminal 15 to go more negative with respect to ground. This voltage difference between generator terminal 15 and ground is sensed and integrated by operational amplifier 181. If this condition continues, the output of operational amplifier 181 continues to integrate in the positive direction making inverting input 159 of operational amplifier 156 more positive which will eventually trip field relay 26 and deenergize shunt field winding 13 of local generator 5.

1n the inputs to operational amplifier 181, there are preferably provided Zener diodes 187 and 192 to raise the common level of voltage to the operating input level of operational amplifier 181 and to provide a predetennined voltage difference between the inputs. Since operational amplifier 181 will begin to integrate in the positive direction whenever the voltage level to inverting input 185 is more negative than the voltage level to noninverting reference input 186, Zener diodes 187 and 192 are selected to allow for a predetermined amount of current to be supplied by local generator 5, and, therefore, a predetermined voltage difference between generator terminal 15 and ground, before input terminal 185 becomes negative with respect to input terminal 186. The Zener voltage of Zener diode 187 is thus chosen to be higher than the Zener voltage of Zener diode 192 by an amount equal to the voltage difference between generator terminal 15 and ground produced by a predetermined allowable amount of current flow through interpole winding 12. When the voltage drop exceeds this amount, however, the voltage level at inverting input 185 of operational amplifier 181 becomes negative with respect to the level of noninverting input 186 and operational amplifier 181 integrates in a positive direction as intended.

The output of operational amplifier 181 is also fed through conductor 194 to the inverting input 129 of operational amplifier 130 of the automatic line contactor control circuit. As previously disclosed, this signal will lock out main contactor 19 and prevent the connection of local generator 5 to load bus L in the event of a local ground fault.

Contact 188 is a normally closed contact of start fault relay 28 which, as previously disclosed, is energized with the main contactor coil. When main contactor 19 is actuated by the automatic line contactor control circuit, start fault relay 28 will also be actuated, causing contact 188 to open so that operational amplifier 181 will no longer be adapted to respond to voltage difference between generator terminal 15 and ground.

Since operational amplifier 181 is no longer adapted to sense a local ground fault when local generator 5 is connected to load bus L, that function is now performed by current transformers 22 and 23 and their associated circuitry. In a typical use of the generator system herein disclosed, each local generator 5 may be located physically a considerable distance from the point where it is connected by main contactor 19 to load bus L so that a considerable length of heavy cable is required. The cable can develop ashort circuit and/or become grounded at any point along its route due to any of a number of causes. To monitor this cable for such local faults, current transformers 22 and 23 are inductively associated in the manner shown with the generator current path at widely spaced locations from each other and thereby provide a protected zone between them; In FIG. 6 of the drawings, the desired interconnection of the transformers is shown by the dot symbol convention applied to the secondary windings of current transfonners 22 and 23 together with the arrows indicating direction of current flow in the primary or load current conductor. Current transformer 23 is placed to sense a change in the current in the return path to local generator 5 while current transformer 22 is placed to sense a change in the generator current at a point in the cable as near as possible to line contactor 19. Both current transformers 22 and 23 should sense the same amount of change. However, if some local fault has developed in the zone protected by current transformers 22 and 23, a current change occurs in one portion of the line which is not balanced by a corresponding current change in the other portion of the line. This unbalanced condition is sensed by the current transformers and translated into a differential voltage developed across resistor 195. Resistor 195 is connected across a rectifier bridge 196 which, in turn, provides a directed gate signal to SCR 197. The cathode of SCR 197 is connected to ground. lts anode is connected through Zener diode 198 to trip coil 30 of field relay 26. A differential voltage of either polarity appearing across resistor 195 is conducted by bridge 196 as a positive signal to the gate of SCR 197. If the signal is of sufficient magnitude, SCR 197 is gated on, tripping field relay 26 and disconnecting local generator 5 from the parallel system. Resistor 199 and capacitor 200 serve as a noise filter to prevent the turn on of SCR 197 by a spurious noise signal.

The foregoing description of a preferred embodiment of this invention is divided into sections and the embodiment, therefore, is treated in sections. This has been done this way as a convenience in presenting and describing the whole embodiment. The various sections or subcombinations work and operate together as an integrated whole. While the subcombinations themselves are novel and enjoy their own features and advantages as generator control and protection subcombinations for a parallel DC generator system, they are also novel when taken together and as a total system for controlling and protecting a parallel system of DC generators and, in their total combination, they enjoy various novel features and advantages as is pointed out in the foregoing description.

Those skilled in the art will appreciate that various changes and modifications can be made in the apparatus described herein without departing from the spirit and scope of the invention.

lclaim:

1. In the method of regulating the output voltage of a DC generator by energizing the field in accordance with the output voltage of the generator the improvement which comprises periodically energizing the field for lengths of time varying in accordance with generator output voltage and at intervals that are substantially shorter than the response time of the generator to applied or removed field excitation and are substantially independent of variations in the output voltage of the generator whereby the mechanical system of the generator is incapable of following the individual energization signals.

2. The method as defined in claim 1 wherein the field is energized at intervals about U l0 of the response time of the generator to applied or removed field excitation.

3. The method of regulating the output voltage of a DC generator connected in parallel with an additional DC power source and driven by a prime mover through an energy absorbing shaft which comprises periodically energizing the generator field for lengths of time varying in accordance with generator output voltage and at intervals that are substantially less than the response time of the shaft to a torsional input produced by applied or removed field excitation and are substantially independent of variations in the output voltage of the generator, whereby the shaft is unable to follow the individual load variations produced by periodic field energization and deenergization and said load variations are attenuated in said shaft.

4. The method as defined in claim 3 wherein the field is energized at intervals about 1/ of the response time of the shaft to a torsional input.

5. In a power system including a DC generator, said generator having an output providing an output voltage and a field winding, a prime mover, an energy-absorbing shaft connected between said generator and said prime mover for transmitting torque therebetween, and a DC power source connected in parallel with the output of said generator, a circuit for controlling the field excitation of said DC generator to regulate its output voltage, said circuit comprising means for sensing the output voltage level of said generator and providing an output voltage signal proportional thereto,

reference voltage means for providing a reference voltage proportional to a predetermined generator output voltage,

means for providing a periodic alternating waveform signal, the period of said signal being substantially less than the response time of said shaft to a torsional input produced by applied or removed field excitation,

static means for algebraically combining said generator output voltage signal and said periodic alternating waveform signal to provide an output signal comprising a periodic alternating waveform superimposed upon a direct current voltage level proportional to the output voltage of said generator so that, by comparison with said reference voltage, the level of said alternating waveform falls below the level of said reference voltage with a frequency corresponding to the frequency of said alternating waveform and for greater and lesser periods of time dependent upon the relative levels of said direct current voltage upon which it is superimposed and said reference voltage,

static-comparing and bistable switching means having a first input connected to said static means and a second input connected to said voltage reference means and having output means for connecting and disconnecting said field winding and said generator output to control the excitation of said generator when said bistable means assumes one and the other, respectively, of its stable states, said comparing and bistable switching means being responsive to said input signals to assume said one of its stable states when said alternating waveform voltage falls below said reference voltage and to assume said other of its stable states when said alternating wavefonn voltage exceeds said reference voltage, said comparing and bistable switching means assuming one or the other of its stable states at intervals substantially independent of variations in the output voltage of said generator whereby said field winding is provided pulses of excitation current at a frequency corresponding to the frequency of said alternating waveform voltage and of shorter and longer duration as the output voltage of said generator exceeds and falls below, respectively, the level of said reference voltage,

said static-comparing and bistable switching means comprising,

an operational amplifier,

a static power switch directly controlling current flow from the output of said controlled generator to said field windmg,

a static control switch connected to the output of said operational amplifier and connected in controlling relationship to said static power switch,

said operational amplifier having an inverting and a noninverting input as said first and second inputs, respectively, and an output providing positive and negative square wave output signals when said inverting input level is less and more, respectively, than said noninverting input level,

said control switch being responsive to the output of said operational amplifier whereby said control switch conducts when the output of said operational amplifier is negative and blocks when the output of said operational amplifier is positive, and

said power switch being responsive to the state of said control switch whereby said power switch conducts excitation current to said field winding when said control switch blocks, and blocks excitation current to said field winding when said control switch conducts.

6. The circuit as defined in claim 5 wherein the period of said alternating waveform signal is about 1/ 10 of the response time of said shaft to a torsional input.

7. in a power system including a DC generator, said generator having an output providing an output voltage and a field winding, a prime mover, an energy-absorbing shaft connected between said generator and said prime mover for transmitting torque therebetween, and a DC power source connected in parallel with the output of said generator, a circuit for controlling the field excitation of said DC generator to regulate its output voltage, said circuit comprising means for sensing the output voltage level of said generator and providing an output voltage signal proportional thereto, reference voltage means for providing a reference voltage proportional to a predetermined generator output voltage, 7

means for providing a periodic alternating waveform signal, the period of said signal being substantially less than the response time of said shaft to a torsional input produced by applied or removed field excitation,

static means for algebraically combining said generator output voltage signal and said periodic alternating waveform signal to provide anoutput signal comprising a periodic alternating wavefonn superimposed upon a direct current voltage level proportional portional to the output voltage of said generator so that, by

comparison with said reference voltage, the level of said alternating waveform falls below the level of said reference voltage with a frequency corresponding to the frequency of said alternating waveform and for greater and lesser periods of time dependent upon the relative levels of said direct current voltage upon which it is superimposed and said reference voltage,

static-comparing and bistable switching means having a first input connected to said static means and a second input connected to said voltage reference means and having output means for connecting and disconnecting said field winding and said generator output to control the excitation of said generator when said bistable means assumes one and the other, respectively, of its stable states, said comparing and bistable switching means being responsive to said input signals to assume said one of its stable states when said alternating waveform voltage falls below said reference voltage and to assume said other of its stable states when said alternating waveform voltage exceeds said reference voltage whereby said field winding is provided pulses of excitation current at a frequency corresponding to the frequency of said alternating waveform voltage and of shorter and longer duration as the output voltage of said generator exceeds and falls below, respectively, the level of said reference voltage,

said static-comparing and bistable switching means comprising an operational amplifier,

a static power switch directly controlling current fiow from the output of said controlled generator to said field winding,

a static control switch connected to the output of said 9. A circuit for controlling the field excitation of a DC generator to regulate its output voltage, said circuit having an output providing an output voltage and a field winding, said circuit comprising operational amplifier and connected in controlling relameans for sensing the output voltage level of said generator tionship to said static power switch, and providing an output voltage signal proportional said operational amplifier having an inverting and a noninthereto,

verting input as said first and second inputs, respectively, reference voltage means for providing a reference voltage and an output providing positive and negative square proportional to a predetermined generator output voltwave output signals whlen staid inverting input level is less "1: 5:81 for providing a periodic Remains avef l d more, es ctive an said noninvertin in ut a W 518m! fe vel, r pc y g p static means for algebraically combining said generator outsaid control switch being responsive to the output of said Pf" Voltage signal and said p f 8 waveforfn 353:? 13512:; ":fi g g ggggg', 2135?,53'; I5 31%;:xingilfifinliitiifii:fifiiFi'iiiFZfiiSli w en u o and of amp I ier is positive, an said power switch being responsive to thfl statei of said con re fl g gfi 1:22:12 3:825:33?g ffzgzgfg ct trol switch whereb said wer switc con ucts excitation current to said f ield wiiiiiing when said control switch responding to the frequency said alternating Waveform blocks, and blocks excitation current to said field winding 32 f0: gtreattler arid letsser gegodstof tlme dteperligent upon when said control switch conducts. 8 re a We eve s 0 Sal cunen 8 P 8. In a system of DC generators having output tenninals and which i is Superimposed and said reference vohagev an equalizer bus terminal, said DC generators being cong 'f d t ll tfl f nected in parallel to a main load bus through their output tera 5 Power 7 con mg ow minals and to each other through connection of their equalizer output of sand controlled generator to Sam field tenninals to an equalizer bus, a circuit for controlling the field excitation of one of said DC generators to regulate its propora Stat: omlrol r f cczmected {he fi. of 'T tionate share of the total load, said circuit comprising 322222??? pz wefgs itzl Comm "'8 means for sensmg. current. flow between controlled said operational amplifier having an inverting and a noningenerator and said equalizer and for providing an equalizer bus voltage signal proportional thereto verting Input as said first and second inputs, respectively, reference voltage means for providing a predetermined and an outwit provldmg P i i .negatwc Square reference signal wave output signals when said inverting input level is less oscillator means for providing a periodic alternating :11 more respectively than smd nomnvemng Input waveform signal, static for algebraisallv Whining said. equalizer bus T ZEZLL'JTIKLIi321"ilZiZZTLL$ZZiI 53L? vpltage signal. and Sald penpdlc fl f f 0 ducts when the output of said operational amplifier is slgnal 9 provlde an output. Sgna] compnsmg a penodlc negative and blocks when the output of said operational alternating waveform superimposed upon a direct current amplifier is positive and 7 Voltage level,c"espnding the dnection of equalizmg said power switch being responsive to the state of said con- Curran? between controlled l trol switch whereby said power switch conducts excitaand sad equahzer bus so that wlth said tion current to said field winding when said control switch reference voltage the level alternatmg waveform blocks, and blocks excitation current to said field winding falls below the level of said reference voltage with a when Said control switch conducts, frequency correspondmg to the frequency of said operational amplifier power requirements being pronfmng waveform and for grefner and pfmods of vided from the output of said DC generator together with dependent p f l level ofsald dn'ect f fail-safe means for insuring the continuous excitation of Voltage "P supenmposed and Sam said DC generator in the event its output voltage falls reference voltage below said operational amplifier requirements, said failstatic-comparing and bistable switching means having a first Safe means including input connected to said Static means and a Second Input a Zener diode having its anode connected to the output of connected to Said reference Voltage means and having said operational amplifier and its cathode to said conoutput means for connecting and disconnecting said field "0| switch and the output f Said DC generator winding and Said general-0r Output to comml the excita' whereby said Zener diode blocks and said DC generator tion of said controlled DC generator when said bistable output vomgc turns ff i comm] switch thereby m ans assumes n and th other, respectively of its turning on said power switch and supplying excitation ble States, Said comparing and bistabk switching means current to said field winding when said operational ambeing responsive to said input signals to assume one of its plifier output i iti e, d, stable states when said alternating waveform voltage falls whereby said Zener diode conducts preventing said DC below said reference voltage and to assume said other of generator output voltage from turning ofi said power its stable states when said alternating waveform voltage switch and preventing excitation current to fiow to said exceeds said reference voltage, said comparing and bistawinding when said operational amplifier output is negable switching means assuming one or the other of its stative and, ble states at intervals substantially independent of variawhereby said Zener diode provides a fixed amount cortions in the output voltage of said generator responding to its Zener voltage by which said DC whereby said field winding is provided pulses of excitation generator output voltage must exceed said operational current at a frequency corresponding to the frequency of amplifier output voltage to permit said control switch said alternating waveform and of shorter and longer durato conduct thereby deenergizing said field winding such tion as said controlled DC generator assumes more and that as said DC generator output voltage falls below the less, respectively, of its proportionate share of system power requirements of said operational amplifier causload on said main load bus. ing its output to become negative, said DC generator t 1 O I 

1. In the method of regulating the output voltage of a DC generator by energizing the field in accordance with the output voltage of the generator the improvement which comprises periodically energizing the field for lengths of time varying in accordance with generator output voltage and at intervals that are substantially shorter than the response time of the generator to applied or removed field excitation and are substantially independent of variations in the output voltage of the generator whereby the mechanical system of the generator is incapable of following the individual energization signals.
 2. The method as defined in claim 1 wherein the field is energized at intervals about 1/10 of the response time of the generator to applied or removed field excitation.
 3. The method of regulating the output voltage of a DC generator connected in parallel with an additional DC power source and driven by a prime mover through an energy absorbing shaft which comprises periodically energizing the generator field for lengths of time varying in accordance with generator output voltage and at intervals that are substantially less than the response time of the shaft to a torsional input produced by applied or removed field excitation and are substantially independent of variations in the output voltage of the generator, whereby the shaft is unable to follow the individual load variations produced by periodic field energization and deenergization and said load variations are attenuated in said shaft.
 4. The method as defined in claim 3 wherein the field is energized at intervals about 1/10 of the response time of the shaft to a torsional input.
 5. In a power system including a DC generator, said generator having an output providing an output voltage and a field winding, a prime mover, an energy-absorbing shaft connected between said generator and said prime mover for transmitting torque therebetween, and a DC power source connected in parallel with the output of said generator, a circuit for controlling the field excitation of said DC generator to regulate its output voltage, said circuit comprising means for sensing the output voltage level of said generator and providing an output voltage signal proportional thereto, reference voltage means for providing a reference voltage proportional to a predetermined generator output voltage, means for providing a periodic alternating waveform signal, the period of said signal being substantially less than the response time of said shaft to a torsional input produced by applied or remOved field excitation, static means for algebraically combining said generator output voltage signal and said periodic alternating waveform signal to provide an output signal comprising a periodic alternating waveform superimposed upon a direct current voltage level proportional to the output voltage of said generator so that, by comparison with said reference voltage, the level of said alternating waveform falls below the level of said reference voltage with a frequency corresponding to the frequency of said alternating waveform and for greater and lesser periods of time dependent upon the relative levels of said direct current voltage upon which it is superimposed and said reference voltage, static-comparing and bistable switching means having a first input connected to said static means and a second input connected to said voltage reference means and having output means for connecting and disconnecting said field winding and said generator output to control the excitation of said generator when said bistable means assumes one and the other, respectively, of its stable states, said comparing and bistable switching means being responsive to said input signals to assume said one of its stable states when said alternating waveform voltage falls below said reference voltage and to assume said other of its stable states when said alternating waveform voltage exceeds said reference voltage, said comparing and bistable switching means assuming one or the other of its stable states at intervals substantially independent of variations in the output voltage of said generator whereby said field winding is provided pulses of excitation current at a frequency corresponding to the frequency of said alternating waveform voltage and of shorter and longer duration as the output voltage of said generator exceeds and falls below, respectively, the level of said reference voltage, said static-comparing and bistable switching means comprising, an operational amplifier, a static power switch directly controlling current flow from the output of said controlled generator to said field winding, a static control switch connected to the output of said operational amplifier and connected in controlling relationship to said static power switch, said operational amplifier having an inverting and a noninverting input as said first and second inputs, respectively, and an output providing positive and negative square wave output signals when said inverting input level is less and more, respectively, than said noninverting input level, said control switch being responsive to the output of said operational amplifier whereby said control switch conducts when the output of said operational amplifier is negative and blocks when the output of said operational amplifier is positive, and said power switch being responsive to the state of said control switch whereby said power switch conducts excitation current to said field winding when said control switch blocks, and blocks excitation current to said field winding when said control switch conducts.
 6. The circuit as defined in claim 5 wherein the period of said alternating waveform signal is about 1/10 of the response time of said shaft to a torsional input.
 7. In a power system including a DC generator, said generator having an output providing an output voltage and a field winding, a prime mover, an energy-absorbing shaft connected between said generator and said prime mover for transmitting torque therebetween, and a DC power source connected in parallel with the output of said generator, a circuit for controlling the field excitation of said DC generator to regulate its output voltage, said circuit comprising means for sensing the output voltage level of said generator and providing an output voltage signal proportional thereto, reference voltage means for providing a reference voltage proportional to a predetermined generator output voltage, means for providing a periodic alternatinG waveform signal, the period of said signal being substantially less than the response time of said shaft to a torsional input produced by applied or removed field excitation, static means for algebraically combining said generator output voltage signal and said periodic alternating waveform signal to provide an output signal comprising a periodic alternating waveform superimposed upon a direct current voltage level proportional to the output voltage of said generator so that, by comparison with said reference voltage, the level of said alternating waveform falls below the level of said reference voltage with a frequency corresponding to the frequency of said alternating waveform and for greater and lesser periods of time dependent upon the relative levels of said direct current voltage upon which it is superimposed and said reference voltage, static-comparing and bistable switching means having a first input connected to said static means and a second input connected to said voltage reference means and having output means for connecting and disconnecting said field winding and said generator output to control the excitation of said generator when said bistable means assumes one and the other, respectively, of its stable states, said comparing and bistable switching means being responsive to said input signals to assume said one of its stable states when said alternating waveform voltage falls below said reference voltage and to assume said other of its stable states when said alternating waveform voltage exceeds said reference voltage whereby said field winding is provided pulses of excitation current at a frequency corresponding to the frequency of said alternating waveform voltage and of shorter and longer duration as the output voltage of said generator exceeds and falls below, respectively, the level of said reference voltage, said static-comparing and bistable switching means comprising an operational amplifier, a static power switch directly controlling current flow from the output of said controlled generator to said field winding, a static control switch connected to the output of said operational amplifier and connected in controlling relationship to said static power switch, said operational amplifier having an inverting and a noninverting input as said first and second inputs, respectively, and an output providing positive and negative square wave output signals when said inverting input level is less and more, respectively, than said noninverting input level, said control switch being responsive to the output of said operational amplifier whereby said control switch conducts when the output of said operational amplifier is negative and blocks when the output of said operational amplifier is positive, and said power switch being responsive to the state of said control switch whereby said power switch conducts excitation current to said field winding when said control switch blocks, and blocks excitation current to said field winding when said control switch conducts.
 8. In a system of DC generators having output terminals and an equalizer bus terminal, said DC generators being connected in parallel to a main load bus through their output terminals and to each other through connection of their equalizer terminals to an equalizer bus, a circuit for controlling the field excitation of one of said DC generators to regulate its proportionate share of the total load, said circuit comprising means for sensing current flow between said controlled generator and said equalizer and for providing an equalizer bus voltage signal proportional thereto, reference voltage means for providing a predetermined reference signal, oscillator means for providing a periodic alternating waveform signal, static means for algebraically combining said equalizer bus voltage signal and said periodic alternating waveform signal to provide an output signal comprising a periodic alternating waveform superimposed upon A direct current voltage level corresponding to the direction of equalizing current flowing between said controlled DC generator and said equalizer bus so that, by comparison with said reference voltage, the level of said alternating waveform falls below the level of said reference voltage with a frequency corresponding to the frequency of said alternating waveform and for greater and lesser periods of time dependent upon the relative levels of said direct current voltage upon which it is superimposed and said reference voltage, static-comparing and bistable switching means having a first input connected to said static means and a second input connected to said reference voltage means and having output means for connecting and disconnecting said field winding and said generator output to control the excitation of said controlled DC generator when said bistable means assumes one and the other, respectively, of its stable states, said comparing and bistable switching means being responsive to said input signals to assume one of its stable states when said alternating waveform voltage falls below said reference voltage and to assume said other of its stable states when said alternating waveform voltage exceeds said reference voltage, said comparing and bistable switching means assuming one or the other of its stable states at intervals substantially independent of variations in the output voltage of said generator whereby said field winding is provided pulses of excitation current at a frequency corresponding to the frequency of said alternating waveform and of shorter and longer duration as said controlled DC generator assumes more and less, respectively, of its proportionate share of system load on said main load bus.
 9. A circuit for controlling the field excitation of a DC generator to regulate its output voltage, said circuit having an output providing an output voltage and a field winding, said circuit comprising means for sensing the output voltage level of said generator and providing an output voltage signal proportional thereto, reference voltage means for providing a reference voltage proportional to a predetermined generator output voltage, means for providing a periodic alternating waveform signal, static means for algebraically combining said generator output voltage signal and said periodic alternating waveform signal to provide an output signal comprising a periodic alternating waveform superimposed upon a direct current voltage level proportional to the output voltage of said generator so that, by comparison with said reference voltage, the level of said alternating waveform falls below the level of said reference voltage with a frequency corresponding to the frequency of said alternating waveform and for greater and lesser periods of time dependent upon the relative levels of said direct current voltage upon which it is superimposed and said reference voltage, an operational amplifier, a static power switch directly controlling current flow from the output of said controlled generator to said field winding, a static control switch connected to the output of said operational amplifier and connected in controlling relationship to said static power switch, said operational amplifier having an inverting and a noninverting input as said first and second inputs, respectively, and an output providing positive and negative square wave output signals when said inverting input level is less and more, respectively, than said noninverting input level, said control switch being responsive to the output of said operational amplifier whereby said control switch conducts when the output of said operational amplifier is negative and blocks when the output of said operational amplifier is positive, and said power switch being responsive to the state of said control switch whereby said power switch conducts excitation current to said field winding when said control switch blocks, and blocks excitation current to said field winding when said control switch conducts, said operational amplifier power requirements being provided from the output of said DC generator together with fail-safe means for insuring the continuous excitation of said DC generator in the event its output voltage falls below said operational amplifier requirements, said fail-safe means including a Zener diode having its anode connected to the output of said operational amplifier and its cathode to said control switch and the output of said DC generator whereby said Zener diode blocks and said DC generator output voltage turns off said control switch thereby turning on said power switch and supplying excitation current to said field winding when said operational amplifier output is positive, and, whereby said Zener diode conducts preventing said DC generator output voltage from turning off said power switch and preventing excitation current to flow to said winding when said operational amplifier output is negative and, whereby said Zener diode provides a fixed amount corresponding to its Zener voltage by which said DC generator output voltage must exceed said operational amplifier output voltage to permit said control switch to conduct thereby deenergizing said field winding such that as said DC generator output voltage falls below the power requirements of said operational amplifier causing its output to become negative, said DC generator output voltage exceeds said negative output of said operational amplifier by less than Zener voltage of said Zener diode. 